Designers of vehicles, particularly heavy equipment vehicles, tractors, and the like have always had to decide whether traction or maneuverability was the more important quality in a given machine. Tractors at one time were generally not considered for four wheel drive systems because it was believed that deliberate weight distribution and bilaterally independent brakes on the drive wheels would be sufficient for maximum tractive force and directional control. Moreover, there are difficulties in providing a driving force to the front steering wheels of such vehicles. If the steering wheels are given drive capability, it would be advantageous to make the steering wheels larger for greater traction. However, larger front steered wheels interfere with the framework of the vehicle, thereby limiting their usefulness. In addition, conventional differentials required to drive the steered wheels are complex and expensive.
Articulated machines overcome some of these difficulties and have the advantage of having a simple non-steered axle on the front, which can support relatively more weight which is advantageous for front mounted implements or when the machine is configured with a loader assembly. The turning radius of articulated machines is not necessarily better than that of more traditionally shaped four wheel drive tractors.
Generally, all-wheel steer machines where both the front and rear axles are steering axles have a tighter turning radius that similar articulated machines. Another advantage to an all-wheel steer machine is that as a straight machine, it has the same tipping load at full turn as it does when the wheels are pointed straight ahead. Articulated machines in contrast have a lower tipping point load when turned because the back half of the machine is both offset relative to the front wheels and closer to the load. These two factors make the machine less stable axially and works to the detriment of the rear half of the machine in its capacity as a counterweight to the load.
Maneuverability has always been an important characteristic of tractors and similar machines. Tracked earthmoving equipment was for many years the benchmark for turning radii. Crawlers had to steer with clutches and brakes which varied the speed of the tracks relative to one another. When one track was locked, the machine could turn on the spot. Two wheel drive tractors can achieve some of the same effects by locking the brake on the inside drive wheel while steering the steered wheels all the way in that direction. Using the brakes allows a tighter turn, and tractor steering radii are customarily rated with and without brakes. Scuffing occurs when the brakes are used because the steering geometry does not turn the steering wheels tight enough to coordinate with a locked drive wheel brake. Scuffing is worse if the tractor is configured to have four wheel drive for at least four reasons:                1. The front tires are larger diameter and have more aggressive tread than their two wheel drive counter parts in order to take advantage of the fact that they are driven. The consequence of this is that the wheels cannot turn as tightly without interfering with the vehicle frame so they have to slide more to achieve the same tight turn.        2. The enhanced traction of driven front wheels means that when they do slide, they tear up more ground because their enhanced traction resists slipping.        3. The braked rear wheel slides or the other drive wheel is likely to spin as they try to force the front wheels to rotate and this tears up the ground.        4. The problem is worse if four wheel drive is engaged because tractors typically do not have a center differential. Accordingly, the arcs of the front wheels are greater than those of the rear wheels. The front wheels effectively rotate too slowly, and the tighter the turn, the greater their speed difference and therefore the greater the scuffing.        
Although for many years crawler tractors were the most maneuverable by virtue of their ability to pivot around one track, the advent of closed circuit hydrostatic systems allowed for an even tighter turn to be practical. Whereas pivoting around one braked track results in a very short radius turn, a spot turn where one track goes in reverse and the other goes forward is tighter. A compound differential provides a similar counterrotation of the tracks.
The concept of counter rotating drive mechanisms has been applied to wheeled machines, and highly maneuverable zero radius turn machines have found their way into timber and mowing machines and small front end loaders. These machines typically use hydrostatic drives which are modular to allow design flexibility. The utility of a zero turning radius vehicle's capacity for turning in a reverse direction within the length of the vehicle, is great enough that the short comings of the two main machine forms of this category have not prevented their widespread acceptance in various applications.
The two machine forms in the category of dual path hydrostatic drive machines are skid steers and machines which have two drive wheels on one axis which steer and propel the machine, and a caster wheel or wheels on the other end of the machine. Skid steers have roughly bilaterally symmetrical drive systems where the front and rear tires on each side are linked exemplarily by a chain. When the left drive transmission goes slower than the right, a left hand turn is effected. If the left wheels are held still and the right wheels are driven forward, a pivot turn to the left is made. This is analogous to the steering clutches and brakes which provide pivot turns of old style bulldozers. Sometimes, because the left and right transmissions are independent, the wheels on one side are driven backwards and the ones on the right are driven forward, which results in a spot turn. This type of four wheel drive vehicle has exceptional traction considering its wheel size, especially in a straight line. Increasing the wheel size is limited by the short wheel base necessary to reduce the horsepower required to make the skidded turn, the necessity of limiting the damage to the ground during the skid turn, and the cost of tires which wear very quickly on this type of machines. Steering induced hop is also a problem in skid steers.
Machines with dual path transmissions connected with wheels which drive and steer the machine at one end and casters on the other end do not have the same wheel diameter limitation of skid steers. Some of these machines are used in timber harvesting, where their maneuverability is an enormous asset as they drive among standing trees, and commercial lawn mowing where they have gained almost universal acceptance. On a larger scale, self propelled wind rowers are dedicated hay mowing machines. The drawback to dual path machines where the two drive wheels share the load of both steering and driving is that if one drive wheel loses traction either because of soft footing or hillside operation, all of the steering is lost. The fact that the other drive wheel may still have traction is of little or no value because it only pushes the machine in the proper direction by chance. The caster wheels provide neither tractive effort nor steering ability and are there only to support weight.
Much prior art now links the steering wheels to the drive wheels in this dual path type machine. Many of these patents seem aimed at operator ease, but surely directional control is improved and appreciated. There is a greater need for improved traction. Some such vehicles have steering wheels which pivot about vertical axes in place of directionally unstable caster wheels. There is a need for improved traction.
The present invention relates to zero turning radius vehicles in which the steering wheels are both driven and rotated about their vertical axes to control the steering and propulsion forces. The other two wheels roll according to the logic in an electronic controller in a coordinated manner relative to the steered wheels. The two non-steered wheels can at times during sharp turns rotate in opposite directions relative to each other, and one may go backwards when the vehicle goes forward, all as determined by the controller and all under power. It is an object of this invention to define the most efficient and cost effective design for the such a vehicle.